1. Field of the Invention
The present invention relates to a method and an apparatus for magnetic resonance imaging, particularly a method and an apparatus for 3D magnetic resonance imaging.
2. Description of the Prior Art
The principle of the 3D MRI is similar to that of the 2D MRI, where the main differences lie in RF excitation and spatial encoding.
Thin slices were excited in 2D MRI to obtain magnetic resonance signal within the specific thickness per repetition while in 3D MRI a much thicker slab containing information of multiple slices was excited. Also in 3D MRI, in addition to the two-dimensional spatial encoding a spatial encoding gradient GZ is further applied so that those nuclei in the excited slab will have certain phase differences in the Z-direction. Also, as opposed to the 2D reconstruction in 2D MRI a three-dimensional inverse Fourier transform (FT) is performed after the 3D k-space acquisition is completed, giving a 3D MR image space.
Referring to the U.S. Pat. No. 4,958,282, it's an image reconstruction method of the 3D FT type wherein the organization of the excitation and measuring sequences comprises the repetition of macro-sequences. During the macro-sequences, a selection encoding gradient which may have a small number of encoding steps (in particular, smaller than the expected resolution of the images) is applied. The prior patent shows that this approach enables the best possible power and flexibility of the vectorial processors used to perform image reconstruction computations, and the images can then be produced in real time. In conclusion, the method for the reconstruction of images acquired by NMR experiments on a body to be examined, the method comprising for the acquisition of the images: the execution of N macro-sequences, each of the N macro-sequences comprising L sequences, the sequences being designed for the excitation and measurement of NMR signals from the body; at each sequence, P samples of this NMR signal are measured; the method also comprising, in a vectorial computer, for the reconstruction of the images by a 3D FT type imaging method, for which the resolution L along one of the imaging axes of the images is smaller than the resolutions N and P along two other imaging axes of the images, the following steps in the following order: a first computation, over samples of the NMR signal, of N*L*P Fourier transforms with P computation steps, followed by: a second computation over results given by the first computation, of N*L*P Fourier transforms with L computation steps, followed by a third computation, over results given by the second computation, of N*L*P Fourier transforms with N computation steps, the final reconstruction of the images being achieved by the end of the third computation.
Referring to the U.S. Pat. No. 5,166,875, in an MRI system, three-dimensional data is acquired from a specific three-dimensional region of a subject to be examined. An image reconstruction process using two-dimensional Fourier transform is performed for two-dimensional data in one direction of the three-dimensional data. An image reconstruction process using 3D FT is performed for the three-dimensional data. In this case, a two-dimensional image is displayed before a three-dimensional image is displayed. In conclusion, the method for processing two- and three-dimensional data in an MRI system, comprising the steps of: acquiring three-dimensional data having a plurality of two-dimensional data; storing the acquired three-dimensional data; reading out one of the plurality of two-dimensional data from the stored three-dimensional data; reconstructing a two-dimensional image by a two-dimensional Fourier transform process for the read out two-dimensional data; displaying the reconstructed two-dimensional image; reading out the stored three-dimensional data; reconstructing a three-dimensional image by a three-dimensional Fourier transform process for the read out three-dimensional data; and displaying the reconstructed three-dimensional image after the reconstructed two-dimensional image is displayed.
According to the above description, both patents are traditional 3D MRI. In the present invention, we further provide a change in the encoded field of view and a separation gradient to decrease the total execution time of 3D MRI system, and to achieve high efficiency and accuracy.